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.Dd $Mdocdate: November 2 2019 $
.Dt CMS_VERIFY 3
.Os
.Sh NAME
.Nm CMS_verify ,
.Nm CMS_get0_signers
.Nd verify a CMS SignedData structure
.Sh SYNOPSIS
.In openssl/cms.h
.Ft int
.Fo CMS_verify
.Fa "CMS_ContentInfo *cms"
.Fa "STACK_OF(X509) *certs"
.Fa "X509_STORE *store"
.Fa "BIO *indata"
.Fa "BIO *out"
.Fa "unsigned int flags"
.Fc
.Ft STACK_OF(X509) *
.Fo CMS_get0_signers
.Fa "CMS_ContentInfo *cms"
.Fc
.Sh DESCRIPTION
.Fn CMS_verify
verifies the CMS
.Vt SignedData
structure
.Fa cms .
.Fa certs
is a set of certificates in which to search for the signing
certificate(s).
.Fa store
is a trusted certificate store used for chain verification.
.Fa indata
is the detached content if the content is not present in
.Fa cms .
The content is written to
.Fa out
if it is not
.Dv NULL .
.Pp
.Fn CMS_get0_signers
retrieves the signing certificate(s) from
.Fa cms .
It must be called after a successful
.Fn CMS_verify
operation.
.Pp
Normally the verify process proceeds as follows.
.Pp
Initially some sanity checks are performed on
.Fa cms .
There must be at least one signature on the data.
If the content is detached,
.Fa indata
cannot be
.Dv NULL .
.Pp
An attempt is made to locate all the signing certificate(s), first
looking in the
.Fa certs
parameter (if it is not
.Dv NULL )
and then looking in any certificates contained in the
.Fa cms
structure itself.
If any signing certificate cannot be located, the operation fails.
.Pp
Each signing certificate is chain verified using the
.Sy smimesign
purpose and the supplied trusted certificate
.Fa store .
Any internal certificates in the message are used as untrusted CAs.
If CRL checking is enabled in
.Fa store ,
any internal CRLs are used in addition to attempting to look them up in
.Fa store .
If any chain verify fails, an error code is returned.
.Pp
Finally the signed content is read (and written to
.Fa out
if it is not
.Dv NULL )
and the signature is checked.
.Pp
If all signatures verify correctly, then the function is successful.
.Pp
Any of the following
.Fa flags
(OR'ed together) can be passed to change the default verify behaviour:
.Bl -tag -width Ds
.It Dv CMS_NOINTERN
Do not use the certificates in the message itself when
locating the signing certificate(s).
This means that all the signing certificates must be in the
.Fa certs
parameter.
.It Dv CMS_NOCRL
If CRL checking is enabled in
.Fa store ,
then any CRLs in the message itself are ignored.
.It Dv CMS_TEXT
MIME headers for type text/plain are deleted from the content.
If the content is not of type text/plain, an error is returned.
.It Dv CMS_NO_SIGNER_CERT_VERIFY
Do not verify signing certificates.
.It Dv CMS_NO_ATTR_VERIFY
Do not check the signed attributes signature.
.It Dv CMS_NO_CONTENT_VERIFY
Do not check the content digest.
.El
.Pp
One application of
.Dv CMS_NOINTERN
is to only accept messages signed by a small number of certificates.
The acceptable certificates would be passed in the
.Fa certs
parameter.
In this case, if the signer is not one of the certificates supplied in
.Fa certs ,
then the verify will fail because the signer cannot be found.
.Pp
In some cases the standard techniques for looking up and validating
certificates are not appropriate: for example an application may wish to
lookup certificates in a database or perform customised verification.
This can be achieved by setting and verifying the signers certificates
manually using the signed data utility functions.
.Pp
Care should be taken when modifying the default verify behaviour, for
example setting
.Dv CMS_NO_CONTENT_VERIFY
will totally disable all content verification and any modified content
will be considered valid.
This combination is however useful if one merely wishes to write the
content to
.Fa out
and its validity is not considered important.
.Pp
Chain verification should arguably be performed using the signing time
rather than the current time.
However since the signing time is supplied by the signer it cannot be
trusted without additional evidence (such as a trusted timestamp).
.Sh RETURN VALUES
.Fn CMS_verify
returns 1 for a successful verification or 0 if an error occurred.
.Pp
.Fn CMS_get0_signers
returns all signers or
.Dv NULL
if an error occurred.
.Pp
The error can be obtained from
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr CMS_ContentInfo_new 3 ,
.Xr CMS_get0_SignerInfos 3 ,
.Xr CMS_sign 3 ,
.Xr CMS_verify_receipt 3
.Sh STANDARDS
RFC 5652: Cryptographic Message Syntax (CMS),
section 5.1: SignedData Type
.Sh HISTORY
These functions first appeared in OpenSSL 0.9.8h
and have been available since
.Ox 6.7 .
.Sh BUGS
The trusted certificate store is not searched for the signing certificate.
This is primarily due to the inadequacies of the current
.Vt X509_STORE
functionality.
.Pp
The lack of single pass processing means that the signed content must
all be held in memory if it is not detached.
